1. Field of the Invention
The present invention relates to a ceramic sintered body and, more particularly, to an aluminum nitride sintered body, a method of manufacturing the same, and a method of evaluation for the same.
2. Description of the Related Art
Conventionally, since ceramic sintered bodies have heat-resistance and corrosion-resistance properties as well as high strength, the ceramic sintered bodies are used in various fields where these properties can be applied. Among the ceramic sintered bodies, an aluminum nitride (AlN) sintered body has oxidation resistance and high thermal conductivity, and is therefore widely used for an electrostatic chuck for holding a wafer, a ceramic heater for heating a wafer, or other purposes.
However, in a case of an electrostatic chuck or a ceramic heater on which a wafer is mounted, it has been pointed out that dropping of ceramic grains occurs due to a thermal shock generated when the wafer is heated to high temperature, a mechanical shock generated when the wafer is attracted or released, abrasion caused by the thermal expansion of the wafer, or the like. Dropped ceramic grains may get into the interface between the ceramic sintered body and the wafer, hinder the adhesion of the ceramic sintered body and the wafer, and degrade the temperature uniformity of the wafer. Moreover, if dropped ceramic grains adhere to the back face of a semiconductor wafer, the ceramic grains may be carried together with the wafer when the wafer is carried in/out, and may fall onto the surface of another wafer. These ceramic grains may cause a wire break or a short circuit in a wiring pattern during a photolithography process, and may lead to a decrease in yield in semiconductor processes.
In addition, many AlN sintered body products are generally subjected to surface grinding and polishing. In cases of conventional AlN sintered bodies, however, dropping of ceramic grains often occurs when a face of a product is ground or polished, because of a mechanical shock and abrasion generated on the grinded or polished face. Since such dropping of ceramic grains creates surface irregularity on a polished face, the surface roughness of the polished face cannot be sufficiently reduced even after the face has been polished for a long time. Therefore, it is difficult to achieve a mirror finish on the polished face.
For the problems concerning the dropping of ceramic grains in an electrostatic chuck or a ceramic heater, a ceramic substrate is proposed which prevents ceramic grains from dropping from a ceramic sintered body (Japanese Patent Publication Nos. 2002-164422 and 2002-170870). These publications disclose that, to prevent the dropping of ceramic grains, it is preferable that a ceramic sintered body exhibits a characteristic of transgranular fracture rather than a characteristic of intergranular fracture.
When fracture occurs in a ceramic sintered body, however, it simply depends on the strength ratio of a ceramic grain to a grain boundary, whether the transgranular fracture mainly occurs or the intergranular fracture mainly occurs. Therefore, even a material which mainly allows transgranular fracture cannot prevent the dropping of ceramic grains if the intergranular strength is not sufficiently high. That is, unless intergranular bonding is stronger than an applied impact, stress or the like, even a material which mainly allows transgranular fracture cannot prevent grain dropping. In addition, since the dropping of ceramic grains mainly depends on the occurrence of cracks at grain boundaries, it is preferable that the intergranular strength, representing the resistance to grain dropping, is expressed as the resistance to the generation and growth of cracks.
On the other hand, evaluation of intergranular strength is not generally performed on ceramics. There is no example in which intergranular strength is evaluated in relation to the dropping of ceramic grains, particularly for an aluminum nitride sintered body. In addition, there are various methods of measuring the strength of ceramics. It is desired to use a proper and practical evaluation method in accordance with the purpose of preventing the dropping of ceramic grains.